I'm trying to do a game with a lot of levels. And I so that if we use a lot of boxes, etc. it can be laggy on Phone also because Scirra uses HTML5. Is that serious.
And btw: How many levels do you think is enough when I upload the game? 150?
Develop games in your browser. Powerful, performant & highly capable.
Using HTML5 is not the cause of low FPS. Using high textures or a lot of physics objects will eat performance even if you code it in a native engine. Read more here: https://www.scirra.com/manual/134/performance-tips (Of course, HTML5 is a bit slower than native, but only a bit. Nowadays it has native-like performance.)
The number of levels you should make is based on your game design. There's no general answer for that.
Read everything about Construct 2 regarding performance. Some of the things you find about performance issues may be back in 2012 and as time goes on, more powerful devices come out and this means better performance on mobile. It literally does take time for performance to get better and for Construct 2 to be more capable of creating better mobile games.
Even so, using Cocoon.IO and optimizing your game very well, will lead to a "good" experience.